Insomeindividuals,theintervalbetweentheQRScomplexandtheTwaveisabnormallyprolonged,aconditiontermedlong QT syndrome (Fig.16.35 ).SeveralcongenitalformsoflongQTsyndromehavebeenidentifiedinhumansubjects.TwoofthemanygenesthathavebeenidentifiedasabasisforthissyndromearetheHERG gene(aK+ channelgene),locatedonchromosome7,andtheSCN5A gene(aNa+ gene),locatedonchromosome3.PatientswithcongenitalformsoflongQTsyndromemayhaveperiodicepisodesofsyncope(fainting),andapproximately10%ofpediatricsubjectswiththisdisordermaydiesuddenly,withoutanyprecedingsymptoms.Insomeindividualswithsilentionchannelmutations,longQTsyndromemaynotbeevidentuntiladrugistakenthataffectstheionchannelinvolved.Manydrugs,includingseveralantiarrhythmicagents,havebeenidentifiedascausingacquiredlongQTsyndrome.
Atrial fibrillation may be changed to a normal sinus rhythm by drugs that prolong the refractory period. As the cardiac impulse completes the reentry loop, it may then encounter refractory myocardial fibers. When atrial fibrillation does not respond adequately to drugs, electrical defibrillation may be used to correct this condition.
Dramatic therapy is required for ventricular fibrillation. Conversion to a normal sinus rhythm is accomplished by means of a strong electrical current that places the entire myocardium briefly in a refractory state. Techniques have been developed to administer the current safely through the intact chest wall. In successful cases, the SA node again takes over the normal pacemaker function for the entire heart.
The Cardiac Pump
The great amount of work performed by the heart over an individual’s lifetime is impressive. A useful way to
Implantablecardioverter-defibrillator(ICD)deviceshavebeendevelopedtopreventdeathinpatientsinwhomeitherventricularfibrillationorparoxysmalventriculartachycardiahassuddenlydeveloped.Theformerislethalunlessitistreatedimmediately,andthelatteroftenleadstoventricularfibrillationandsuddendeath.TheICDdeviceisimplantedsubcutaneouslyintheleftsubclavicularregionofthechestwall.Atrialandventricularleadsenablerecordingoftherightatrialandrightventricularelectrocardiogramsandprovidetheabilityforrightatrialorrightventricularpacing,orboth.Thedefibrillationcoilintherightventricleenablestheapplicationofastrongelectricalcurrenttotheventricleandtherebyusuallyterminatesthelethalarrhythmia.
understand how the heart accomplishes its important task is to consider the relationships between the structure and function of its components.
Relationship of Heart Structure to Function
The Myocardial Cell ). Of importance is that both are striated as a result of regular arrangement of the contractile proteins actin and myosin, and generation of force and contraction of muscle fiber occur as a result of their interactions (i.e., sliding filament mechanism).
Skeletal muscle and cardiac muscle show similar length-force relationships. As shown in , this relationship for the heart may be expressed graphically, as in
Fig.
16.36, by substituting ventricular systolic pressure for force and end-diastolic ventricular volume for resting myocardial fiber (and hence sarcomere) length. The lower curve in
Fig.
16.36 represents the increment in pressure produced by each increment in volume when the heart is in diastole. The upper curve represents the peak force or pressure developed by the ventricle during systole as a function of initial fiber length (or diastolic filling pressure). This curve illustrates the Frank-Starling law of the heart.